I have a class that's being imported in module_x for instantiation, but first I want to override one of the class's methods to include a specific feature dynamically (inside some middleware that runs before module_x is loaded.

Neither AndiDog's nor Andrew's answer answer your question completely. But they have given the most important tools to be able to solve your problem (+1 to both). I will be using one of their suggestions in my answer:

You will need 3 files:

File 1: myClass.py

class C:def func(self):#do something

File 2: importer.py

from myClass import *
def changeFunc():A = C()A.func = lambda : "I like pi"return Aif __name__ == "importer":A = changeFunc()

File 3: module_x.py

from importer import *
print A.func()

The output of module_x would print "I like pi"

Hope this helps